1. Field of the Invention
The present invention relates to projectors for images, methods of measuring a distance from the projector to a projection screen, and methods and programs for acquiring an inclination angle of the projection screen.
2. Background Art
With a projector which projects an image on a screen, if the screen is not vertical to the optical axis of the projector, the image would be distorted by the screen even when the original image is not distorted. In order to avoid this situation, the projector performs an Automatic Keystone Correction (AKC) on the image, which includes correction of distortion of the image without correcting the inclination of the screen.
There are projectors that include a device which measures an inclination angle of the screen relative to an ideal projection screen perpendicular to the optical axis thereof. These measuring devices use a sensor which operates based on a triangular distance measuring method to measure a plurality of distances each between the projector and a respective one of a like number of points on the screen. These devices calculate the inclination angle of the screen based on the plurality of distance data obtained by the measurement. Then, the projector performs the AKC on the image based on the angle calculated.
For example, Published Unexamined Patent Application 2005-233880 (pages 5, 6; FIGS. 1, 2) discloses such conventional projection screen inclination measuring device using, for example, a passive phase difference distance measuring sensor. This sensor focuses an image on each of a pair of image capturing elements with a lens and then detects a phase difference between the two focused images. This device measures a plurality of distances each between this device and a respective one of a plurality of points on the screen based on the phase difference sensed by the sensor.
With this device, as the distances increase, the measurement accuracy decreases. With a general front type projector, the projection distance is 3-4 m and the conventional projection screen inclination angle measuring device cannot achieve sufficient measurement accuracy for that projection distance.
Since the conventional phase difference sensor requires the lens for focusing images on the respective two image capturing elements, an interval equal to the focal distance is required between the lens and the image capturing elements. Especially, at present, the projectors and cameras sensors are required to be reduced in size, although the phase difference sensors are difficult to miniaturize further.